Add the possibility to provide an artefact directory as absolute path

# Add the possibility to provide an artefact directory as absolute path

Currently in babel-plugin-relay we cannot specify the artefact directory as an absolute path.
- Is a feature we can add if we submit a pull request ? Or it's not expected at all to be able to have this ?

This is the place where we generate the paths.
https://github.com/facebook/relay/blob/fd038a2c7dd777063adce2e52ace2fa65817b08d/packages/babel-plugin-relay/compileGraphQLTag.js#L114

## Motivation to have an absolute path

In some cases (A minority I think) we need an absolute path. Let's say we are in a monorepo and we want to have HMR across the whole monorepo which can be nice, we setup trampling rule to transpile our other packages, so we need a "package import" who is absolute and not a relative import

cc @ThibaultBoursier